December 14, 2009 | David F. Coppedge

Iapetus Mystery Moon Solved

Iapetus, that black-and-white moon orbiting Saturn that has mystified astronomers for 338 years, may finally be understood.  The dark material did come from outside – not inside.  The exogenic theory has won.  It’s been coated with dust from outer moons, but that’s not all: uneven heating has moved the ice around on the surface, accentuating the contrast between light and dark areas.  This combination of processes explains the yin-yang appearance of this moon, with its ten-fold brightness dichotomy that was noticed by Jean-Dominique Cassini, its discoverer, in 1671.
    Two papers published ahead of print on Science Express explain the latest theory.1,2  Jet Propulsion Laboratory issued a short press release about the models based on data from the Cassini orbiter.  A more detailed press release was posted by Southwest Research Institute (SwRI).  PhysOrg presented a summary with graphs.  See 10/16/2007 bullet 3, 05/05/2008, and 10/07/2009 for previous CEH entries about the mysteries of this moon’s surface.  (Note: the current papers do not address or explain the mystery of the equatorial mountain ridge; see 03/01/2006 and 02/06/2006.)
    Iapetus is a victim of a “runaway feedback loop, operating on a global scale,” the SwRI press release states.  Infalling material, probably from the Phoebe ring (see 10/07/2009), accumulates on the leading side of the moon.  Like many moons, Iapetus always keeps one hemisphere facing the planet.  By analogy, if you saw dust on your windshield but not on your rear window, you would assume you were moving forward through a field of dust without spinning the car.  Iapetus thus has a “leading hemisphere” and a “trailing hemisphere.”  The leading hemisphere is the dark one.  But simple infalling dust is not enough to explain the extreme differences in brightness.  All existing spacecraft images of Iapetus from Voyager and Cassini can be found at JPL’s Planetary Photojournal.  Images PIA08375 and PIA08374 are especially striking.
    Because Iapetus has a slow orbit around Saturn (29 years), the dark portions are exposed to sunlight longer than on inner Saturnian moons.  This creates just enough temperature difference in the dark areas to cause the water ice to sublimate.  Then, the low gravity on Iapetus makes the water molecules bounce toward the poles and the cooler trailing hemisphere, leading to a pile-up of bright ice on one face.  The dark material on the “windshield” get darker over time.  A similar process may explain subtle brightness dichotomies on other moons, such as Oberon and Titania at Uranus.  The situation at Iapetus, though, is extreme.
    Critical observations by Cassini that led to these conclusions included the shallow depth of the dark material, the segregation of material on crater walls and extremely sharp boundaries of dark and light material in places, temperature measurements, spectra of the dark material, and observations of infalling material from the outer moons, especially Phoebe (06/14/2004, 02/28/2005, 10/07/2009).  The scientists then modeled the behavior of ice under those conditions.
    The papers gave some clues about the age of the surface.  Bright floors of some craters show that the dark material is only meters thick at most – probably quite a bit less than one meter, according to radar measurements.  Because of the absorbent dark dust, the bright ice heats up and moves around quickly.  Spencer and Denk said, “Because of the extreme temperature dependence of sublimation rates, mean sublimation is determined largely by maximum diurnal temperature rather than mean temperature, so ice on Iapetus with the low albedo of the leading side has by far the highest sublimation rate of ice on any Saturn satellite, equivalent to over 100 meters of sublimation in a billion years if unimpeded by the formation of a lag deposit.”  In their computer model, they assumed deposition rates of 3 and 0.3 cm per billion years.  Their resulting maps matched the actual surface dark-light boundaries pretty well.  The rate of sublimation, however, is very short:

A strong prediction of this model is that the dark material of Iapetus’ Cassini Regio [the dark hemisphere] should be essentially ice-free.  On Iapetus’ dark terrain, with 129 K peak daytime temperatures, 1 mm of ice should sublime in only 8,000 years.  Cassini Regio shows both a weak H2O absorption edge at 160 nm, and a strong 3 [micrometer) H2O band, but these features may be due to bound water rather than H2O ice.  The weaker 1.5 and 2.0 [micrometer] H2O bands typically seen in planetary water ice are not seen in Cassini Regio, so if ice is present it must be in micron-sized grains.  However micron-sized ice grains should sublime away in decades at these temperatures.

The fact that Cassini still sees a faint signature of water ice in the dark material seems to argue that the surface has not been undergoing this runaway segregation for billions of years.  In addition, the second paper by Denk et al noted that bright young craters should darken quickly: “If a new crater has excavated icy material that is ~10X brighter than the dark Cassini Regio coating (and thus about as bright as icy parts of the trailing side), it is only about half as bright ~10,000 years later.”  Yet Cassini Regio (the dark area) is punctuated by bright areas, as the latest high-resolution map indicates.  One of the largest impact basins, a crater named Malun 75 miles across, is completely dark.  “Despite its relative youthfulness,” the authors commented, “its surface is completely dark, arguing for a darkening process that continued after Malun formed.”  Spencer and Denk pointed out also that impacts will excavate and mix ice into the upper layers, exposing it to sublimation.  This process, called “impact gardening,” should accelerate the darkening of the leading hemisphere.
    If thermal segregation has been going on for billions of years, why are any bright features left on the dark side?  By making certain assumptions about impact rate, they produced a graph (Figure 3C in the Denk et al paper) that shows some bright craters as old as 400 million years (less than 1/10 the assumed age of Iapetus); the majority are far younger.  All these, however, should have been darkened completely in the rapid thermal segregation process.  Crater Escremiz in particular is extremely bright and still has prominent bright rays extending from the impact.  Unless they can believe most of these bright young craters were produced in the last 90% to 99% of the assumed age of the solar system, thinking about the age of this moon’s surface seems poised for fresh excavation.


1.  John R. Spencer and Tillman Denk, “Formation of Iapetus’ Extreme Albedo Dichotomy by Exogenically Triggered Thermal Ice Migration,” Science, Published Online December 10, 2009, Science DOI: 10.1126/science.1177132.
2.  Denk et al, “Iapetus: Unique Surface Properties and a Global Color Dichotomy from Cassini Imaging,” Science Express, Published Online December 10, 2009, Science DOI: 10.1126/science.1177088.

For those of us who have followed this mystery since the Voyagers in 1981 failed to resolve it, there is a mix of satisfaction and wonder at the solution.  Who would have thought of this answer before we had the high-resolution images that forced it?  It’s an illustration of how observations – data – are vital for constraining theories, models and speculations (like those of Arthur C. Clarke in 2001: A Space Odyssey, who made Iapetus the Stargate of the advanced aliens; Wikipedia explains).
    Yet there is also a strange sense of schizophrenia in the scientific papers.  On the one hand, the authors cannot avoid the evidence that many of these features look young.  On the other, though, they cannot and will not ever admit that they are too young to fit into the billions-of-years paradigm.  The papers go strangely silent on that point.  It’s bizarre.  The assumed Age of the Solar System (A.S.S.) is, in planetary science, a Law of the Misdeeds and Perversions that cannot be altered.  The solution is to either ignore the problem, or to change the subject: i.e., H2O on Iapetus?  Maybe there could be liquid water under the crust – maybe there could be life! (Examples: 03/26/2008, 07/12/2007, bullet 1, PhysOrg.)
    Remember that it’s more reasonable to set an upper limit on age than a lower limit.  Why?  Because the observation-to-assumption ratio is lower.  We only have a few centuries of observation of Iapetus – only a few decades of spacecraft observations, the best observations within the last 4 years from Cassini.  Extrapolating what we know or can reasonably infer back in time by a few decades or centuries or millennia may be acceptable, but not billions of years.  These papers indicate that impact gardening, sublimation and thermal segregation will darken a crater within a few thousand years.  With very liberal assumptions they can stretch some of the bright craters into a few tens or hundreds of millions of years – but not billions.  It’s probably far less, due to the fact that the ice is lost to space each orbit (see 05/05/2008).  But even the most generous upper limit is far less than the time needed to keep Iapetus 4.5 billion years old.
    Lower limits, on the other hand, constrain the upper limits and pull them back closer to young ages.  (The absolute lower limit is Voyager’s arrival year of 1981, when observations of the surface began.)  Suppose a meteor swarm caused a larger number of impacts within the last 10,000 years.  Suppose further that impacts on Phoebe sent extraordinary amounts of dust hurtling toward Iapetus within that time period.  Phoebe, the reputed source of the dark material on Iapetus, is, after all, riddled with large craters showing exposed ice; one is 62 miles across (see Photojournal).  We don’t know how much material from Phoebe and the other retrograde outer moons of Saturn landed on Iapetus, or when it did, or whether it arrived episodically or in steady state.  The large Phoebe craters suggest episodes of heavy deposition occurred.  The presence of the huge Phoebe Ring (10/07/2009), furthermore, suggests that material could pile up quickly on Iapetus – yet only a meter or less is seen today.  Other observations militate against slow-and-gradual accumulation over billions of years: the rapid loss of carbon dioxide ice, the amount and extent of bright material in Cassini Regio, the quick darkening of young crater Malun, and the incomplete segregation in the bright areas.  They certainly make it hard to believe in billions of years.  But if we rule out billions of years, it raises a whole new set of questions the evolutionists dare not entertain.
    What they do, therefore, is tweak their models to preserve the A.S.S.  Spencer and Denk, for instance, assumed dark material deposition rates of 0.3cm and 3.0 cm per billion years.  Why?  Because those rates produce the Iapetus-like pattern in billions of years.  How convenient.  It locks the A.S.S. right into the model (this is known as circular reasoning).  This way, their resulting graphs match the observed pattern in 2.4 billion years (whoops; that’s only half the assumed age of the moon; oh well, close enough?)
    A quick look at their model parameters chart in the Supplementary Material shows they considered a faster deposition rate of 25 cm per billion years, with the comment, “Faster dark material accumulation rate.”  Since that setting produced the pattern in just 300 million years, it was quietly dismissed with this comment: “Model E shows the effects of increasing dark material accumulation rates.  For this model, darkening of mid-latitudes on the trailing hemisphere happens more quickly than sublimation can brighten them, resulting in much greater darkening of the entire leading hemisphere than is seen on Iapetus, in just a few hundred million years.”  Well, then, maybe it could produce the observed pattern in a lot less than a few hundred million years.  They left themselves some leeway due to the “crudeness of our model.”
    Models by definition are only simulations of reality.  Other plausible models with variable deposition rates and impact rates might produce the observed pattern in orders of magnitude less time.  A rigorous examination of the assumptions used in these papers, constrained by observations, might rule out the billions of years evolution requires.  Here’s an opportunity for creation scientists to tighten the upper-limit belt for the processes on Iapetus, and cause some impact on the evolutionary A.S.S.

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Categories: Physics, Solar System

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